The technical field of this application is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors. Characterization of bulk material condition includes (1) measurement of changes in material state, i.e., degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from aggressive grinding, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or with single or multiple cracks, cracks or stress variations in magnitude, orientation or distribution.
A specific application of these techniques is the inspection of high-strength steel components with the goal of measuring applied and residual stresses and detecting early stage fatigue damage or hydrogen embrittlement. Highly stressed aircraft components, such as landing gear components, require the use of steels such as 4340M and 300M heat treated to very high strength levels. The integrity of these components is critical to the safe operation of aircraft and for maintaining readiness of military aircraft. However, unintentional loading of these components, such as a hard landing, can impart residual stresses that compromise the integrity of the component. Similarly, the mechanical properties of these ultra-high strength steels can be seriously degraded as a result of the ingress of hydrogen. Hydrogen ingress can occur during pickling or plating operations and also during cleaning with citric acid based maintenance solutions. The resulting hydrogen embrittlement is unpredictable and can cause catastrophic failure of the component. Hydrogen embrittlement has been established as the direct cause of numerous landing gear failures. This similarly applies to related degradation mechanisms such as temper embrittlement, creep and other degradation processes that reduce a materials functional behavior.
The detrimental effects of hydrogen on material properties and component integrity have been observed in a wide range of metals, as described for example in Interrante and in Hydrogen in Metals. Management of high-strength steel components embrittled by hydrogen is made more difficult by the fact that failures are typically delayed, occurring some time after ingress of atomic hydrogen. The delay between exposure to hydrogen and failure of a high strength steel component depends on a number of factors. Among these are the levels of hydrogen concentration, tensile stress, temperature, stress gradients, and certain impurities in the steel, as well as the type, concentration, and size of certain crystal lattice defects and inclusions. Moreover, susceptibility to hydrogen embrittlement can vary significantly between different heats of steels and between different pours from a given heat, as described by Lawrence. Hydrogen concentration on the order of a few parts per million is sufficient to cause hydrogen embrittlement and delayed fracture. Once atomic hydrogen enters the steel, excess hydrogen atoms diffuse to inclusions, preexisting defects, and zones of high dislocation density. Some hydrogen atoms, as a result of stress-assisted diffusion, can cluster and form “platelets” leading to initiation of microcracks. When such platelets form in front of a crack tip, they facilitate crack extension. Critical regions where hydrogen cracks are more likely to initiate are notches or other stress raisers where local hydrogen concentration is higher due to enhanced diffusion into the triaxially stressed region in front of a stress raiser. Cracks at these critical locations often initiate close to but beneath the surface, making them more difficult to detect.
A recent review of existing magnetic/electromagnetic, diffraction, ultrasonic and other methods for assessment of residual stresses in steel components by Bray highlighted strengths and weaknesses of the available methods. This review also indicated that practical and cost-effective methods for assessment of residual stresses as well as for monitoring of applied stresses over wide areas in steel components are not yet available. Typically, discrete strain gages are mounted directly onto the material under test (MUT). However this requires intimate fixed contact between the strain gage and the MUT and individual connections to each of the strain gages, both of which limit the potential usefulness for monitoring stress over large areas. Furthermore, strain gages are limited in durability and do not always provide sufficient warning of gage failure or malfunction. Possible correlations between magnetic properties and stresses in ferromagnetic materials have been studied for over 100 years, as reviewed by Bozorth. Magnetostriction effect data suggests that, depending on the magnitude and sign of the magnetostriction coefficient, correlation between stress and magnetic permeability within certain ranges of the magnetic field should be present. However, attempts to use conventional inductive, i.e., eddy-current sensors for assessment of residual stresses as well as for a number of other applications have shown serious limitations, particularly for complex geometry components.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks. Conventional eddy-current sensors widely used in nondestructive testing applications are effective at examining near surface properties of materials, but have a limited capability to examine material property variations deep within a material. In contrast, ultrasonic techniques that are also widely used are effective at measuring property variations deep within a material, but have limited sensitivity near the surface and behind some geometric features such as air gaps.